CHARACTERISATION AND MODELLING OF THE FATIGUE DAMAGE AND FAILURE IN UNIDIRECTIONAL COMPOSITE

This PhD thesis is the results of a three-year work aimed at improving the knowledge on the micromechanics of damage formation and evolution on composite laminates subject to fatigue loadings. Fatigue damage affects the elastic properties and could lead to out-of-specification deformations. Being able to predict the damage evolution at the lamina scale enables an improved design and development of composite components. The first research topic concerns the characterization of fatigue crack initiation and propagation in laminates, with particular emphasis on the variable amplitude block and spectrum loadings, common in real structural applications. The effects of variable amplitude loadings were analysed experimentally from macro-a micro scale perspective. Based on the obtained observations, a modelling approach to deal with generic spectrum loadings was developed. The second research line was about delaminations in laminates, induced by the presence of off-axis cracks. Experimental damage analyses were carried out on a specifically designed specimen configuration. The phenomenon was then simulated using Finite Elements and a fracture mechanics approach. Developing damage evolution models requires to know the stress fields in plies in the presence of damage (cracks and delaminations), so that the last research area consisted in the development of a stress distribution model for generic laminates subject to in-plane and bending loads. The same model can be used for also estimating the full stiffness matrix of laminates in the presence of cracks.